This invention relates to vibrating screens and more particularly to suspension and damping systems for vibrating screens.
The aggregate industry utilizes many styles of screen machines to sort aggregates by size. Most screen machines utilize vibration to agitate the mixture of aggregates to promote separation through various sized openings in the screening surfaces. Sorting is achieved by undersized particles passing through the openings in the screening surface and the oversize particles being retained above the screen surface. These machines usually have some type of vibrating mechanism to shake the unit and its screening surfaces. The vibrating mechanisms usually include an unbalanced weight mounted on one or several rotating shafts which, when rotated, force a cycling motion into the screen machine. The resulting motion can have a circular path, linear path, elliptical path, or any combination of those shapes. This cycling motion is referred to as the screen stroke and can range in total displacement in any direction from less than ¼″ to more than 1″.
These screen machines are normally supported on springs which isolate the vibrating machine from the mounting structure.
Fixed inclined screens are constructed so the screen surfaces are sloped, usually toward the discharge end, to aid material movement to the end and off the screen. These vibrating screens are usually supported with four springs or spring groups, one each at the corners of the screen. The springs are usually mounted in a vertical orientation.
Sometimes a screen is designed to be operated in various sloped positions. This is frequently found in portable equipment that requires a lower profile for travel as well as multiple sloped positions as needed for various screening applications.
Now referring to FIG. 1, in the case of a screen that must operate at various sloped positions, vertically mounted springs 14 and 16 would become tilted with the change of slope and become unstable. Therefore, the springs in this case are typically oriented so they are tilted towards each other within the spring grouping 10 to provide spring stability as the support frame 12 changes slope. These are also commonly found in two spring groups, or in an alternate arrangement with a center vertical spring.
The overall spring rate, or stiffness, of the spring groups are affected as the support frame changes slope. All the spring groups change together as the slope changes. If the center of gravity of the screen is above a plane that goes through the spring attachment points, there will be a shift of weight to the discharge end of the screen as the slope increases. It would be desired to have the spring groups on that end to increase stiffness to help support the weight shift. Even if the center of gravity is on the plane through the spring attachments, the heavy load of unsorted material on the upper levels will raise the mass center of the screen which will shift more weight to the discharge spring groups as the slope increases.
In order to provide significant isolation from the mounting structure, the spring suspension has a sufficiently low spring rate to minimize vibration transmittance into the mounting structure. The natural frequency of the spring supported machine is lower than the vibration frequency in order to provide isolation. Since the spring natural frequency is lower than the operating frequency, the machine must pass through the natural frequency speed range during start up and shut down. When the machine passes through the suspensions natural frequency range, that motion becomes amplified and the movement of the screen body becomes much larger than the motion (stroke) during normal operation. This large motion or surge causes higher forces and stresses to the screen and support structure which can cause damage to both.
It is desired to employ a mechanism to dampen the surge during that start up and shut down sequence. There are various styles of damping methods used today, most utilizing some type of friction device to dissipate some of the energy during the surge. Most devices used today either require continual maintenance or dampens only vertical motion. There are mechanisms in the industry today that utilize a yoke type containment device and a single pivot link. These only contact the screen body stub post when moving vertically, not horizontally.
Since it is an elongated yoke, it makes point contact on a horizontal surface rather than the rounded surfaces of the containment cup. The yoke style also does not provide horizontal containment. See FIG. 2.
Consequently, there is a need for improvement in suspension and damping systems for vibrating screens.